Method for cleaning semiconductor devices

ABSTRACT

A method is provided for treating a plurality of semiconductor substrates using the same aqueous SC-1 solution which solution removes and/or inhibits contamination of the semiconductor surfaces by metallic ions present in the solution or on the substrate surface comprising a basic solution containing hydrogen peroxide and an oxidation-resistant chelating additive such as CDTA in an amount effective to provide the desired treatment results. The SC-1 solution may be the conventional 5:1:1 (water:NH 4  OH:H 2  O 2 ) solution or a dilute solution such as a 5:x:1 to 200:x:1 solution wherein x is 0.025 to 2.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to cleaning (treating) semiconductorsubstrate surfaces during the electronic component fabrication processand, in particular, to a method for cleaning the substrate surfacesduring a fabrication process in which a plurality of substrates arecleaned using basic aqueous solutions of hydrogen peroxide containingspecially defined complexing agents which solutions remove organic andparticulate matter from the substrate surfaces and/or inhibit metalcontamination of the substrate surface from the cleaning solution duringthe fabrication process.

2. Description of Related Art

In semiconductor cleaning technology, basic aqueous solutions ofhydrogen peroxide are primarily used to contact and clean semiconductorsto remove organic and particulate matter from the substrate surface.Metallic contaminants and/or organic particulate matter on the substratesurface are a problem and the well-known RCA-cleaning method istypically used for treating the semiconductor surface. In general, theRCA-cleaning method is a two-step wet cleaning cycle in which, in thefirst step, a basic aqueous solution of hydrogen peroxide (generallycalled SC-1 solution) is used to remove particles and organiccontamination on the semiconductor surface. In the second step, amixture of HCl, H₂ O₂ and H₂ O (generally called SC-2) is used to removemetallic contaminants from the semiconductor surface.

One of the major drawbacks of the SC-1 type solutions is thecontamination of the semiconductor surface with metallic ions andprecipitates by adsorption and/or ion-exchange mechanisms duringcontinuous use of the solution over a period of time in which a numberof semiconductor substrates are cleaned in the same cleaning bath. Thismetal contamination has been shown to be responsible for certain defectformation mechanisms in subsequent device manufacturing steps. The metalcontamination occurs as a result of metal ions in the SC-1 cleaningsolution. The sources of the contaminating ions are impurities in theprocess chemicals and/or the treated wafers. Another critical problem,especially when a hydrophobic silicon surface is immersed in such amixture, is the possibility of surface roughening by micro-masking byoxygen bubbles generated through peroxide decomposition and throughlocalized corrosion (triggered by adsorbed metallic ions orprecipitates).

To remove this metallic contamination from the surface before the nextprocessing step, the additional acid cleaning step using the SC-2cleaning solution is performed. This removes the metallic contaminationefficiently, however, re-contamination with particles often occurs. Inorder to avoid these problems, it is desirable to eliminate the SC-2acid cleaning step for a variety of reasons including cost, equipmentsize and cycle time reduction. To eliminate the SC-2 step, it isnecessary, however, to reduce or inactivate the metal contaminants inthe SC-1 cleaning step to below a specified maximum allowable activitylevel.

Complexing or chelating agents are known to have been added to the basicSC-1 cleaning solution to reduce the residual metal contamination on thesemiconductor surface after the cleaning step. This has been shown in anumber of patents including U.S. Pat. Nos. 5,290,361 and 5,302,311;European Patent Nos. 92906248.7 and 93103841.8; Japanese Patent No.PCT/JP92/00219 and German Patent No. DE3822350A1. In U.S. Pat. No.5,290,361, a phosphonic acid complexing agent is employed in the SC-1solution to lower or inhibit the metallic impurities left on thesemiconductor surface. The patent discloses the use of a number of otherchelating agents such as ethylenediamine tetraacetic acid (EDTA);1,2-cyclohexanediaminetetraacetic acid (CDTA);triethylenetetraminehexaacetic acid (TTHA); and nitrilotriacetic acid(NTA) as not being as effective as the phosphonic acid complexants ofthe patent. This is also demonstrated in the article "Thin-OxideDielectric Strength Improvement by Adding a Phosphonic Acid ChelatingAgent into NH₄ OH--H₂ O₂ Solution" by Akiya et al., J. Electrochem.Soc., Vol. 141, No. 10, October 1994, pp. L139-L142.

The above references, however, do not address the major concern of afabrication process which is the need for a cleaning solution whichmaintains its cleaning activity over a period of time during which alarge number of semiconductor substrates are cleaned using the samebath. For example, in the aforementioned U.S. Pat. No. 5,290,361, knownaminoacetic acid chelating agents are stated to be ineffective in SC-1cleaning solutions when compared to the claimed phosphonic acidadditives. The cleaning tests performed were concerned, however, onlywith a single cleaning experiment and did not address the more importantproblem of both bath stability and bath effectiveness over time.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide stable cleaningsolutions for cleaning a plurality of semiconductor substrates with thesame solution which solutions clean the surface while removing fromand/or inhibiting metallic contamination of the semiconductor surface.

It is a further object of the present invention to provide a method fortreating semiconductor substrate surfaces with cleaning solutions, inparticular, SC-1 type solutions, which solutions remove from and/orinhibit metallic contamination of the semiconductor surface during thecleaning process in which a plurality of semiconductors are cleanedusing the same solution or a replenished solution, and, further, whichsolutions are stable over extended use periods.

Another object of the present invention is to make the wafer cleaningprocess more robust in relation to accidental metal ion contamination ofthe solution and/or the cleaning equipment containing the solution sincethe presence of the complexant prevents wafer contamination and loweryields. Contaminated cleaning equipment can also be cleaned withoutbeing taken off line by using the cleaning solution of the invention.

Still other objects and advantages of the invention will be readilyapparent from the following description.

SUMMARY OF THE INVENTION

The above and other objects and advantages, which will be apparent toone of skill in the art, are achieved in the present invention which isdirected, in a first aspect, to an aqueous solution for treating aplurality of semiconductor substrate surfaces comprising an organic orinorganic base, hydrogen peroxide and 1,2-cyclohexanediaminetetraaceticacid (CDTA) as an additive in an amount greater than about 1 ppb to1,000 ppm to minimize and/or inhibit contamination of the semiconductorsurface by metallic ions present in the treating solution or on thesemiconductor substrate surface. In a preferred aspect, the CDTA ispresent in an amount of about 10-100 ppm. The pH of the cleaningsolution is about 9-11.

In another aspect of the invention, an aqueous solution is provided fortreating semiconductor substrate surfaces comprising an organic orinorganic base and hydrogen peroxide. In the case of ammonium hydroxideas base, the volume ratio of water:base (ammonium hydroxide):H2O2 isgreater than about 5:x:1 to about 200:x:1 wherein x is about 0.05 to 2and the solution further contains CDTA in an amount of about 1 ppb to1,000 ppm. With regard to the base material, ammonium hydroxide is thetypical base of choice and is generally used in a 25-30% by weightaqueous solution (NH₄ OH). Likewise, hydrogen peroxide is typically usedin the form of a 28-32% by weight aqueous solution. Accordingly,therefore, when a volume ratio such as 5:1:1 is specified as thetreating solution, this will refer hereinafter to the use of the aqueoussolutions of ammonium hydroxide and hydrogen peroxide and not to thepure forms of each material. It will then be understood that, forexample, a 5:1:1 volume ratio is, in effect, a more dilute solution withregard to the amount of base and peroxide actually in the solutionrelative to the water.

In a preferred treating solution, the volume ratio of the components isabout 10:1:1 to about 40:1:1. In highly preferred cleaning solutions,the CDTA is above about 1 ppm to about 100 ppm and most preferably about10 ppm. The CDTA is readily soluble in basic solutions but not in waterand the CDTA can be added to the cleaning solution in solid form or,preferably, as a solution in the base used in the cleaning solution,preferably ammonium hydroxide.

In a further aspect of the invention, a method is provided for treatinga plurality of semiconductor substrate surfaces with the same cleaningsolution or a replenished solution to clean the surfaces while removingfrom and/or inhibiting metallic contamination of the treated surface.The steps comprise contacting a plurality of the semiconductor substratesurfaces with an aqueous solution comprising an organic or inorganicbase, hydrogen peroxide and CDTA in an amount greater than about 1 ppbto about 1,000 ppm. Typically, a number of batches of semiconductorsubstrates are cleaned using the same treating solution or replenishedsolution, and it is an important feature of the invention that thecleaning solution is stable over time and during use of the solution totreat a number of batches of substrates. Each batch typically containsabout twenty-five wafers. It has been found that there is a lowermetallic contamination on the substrates when using the cleaningsolutions of the invention during a typical fabrication process,compared to a SC-1 solution without complexant.

In an additional aspect of the invention, the above method is highlypreferred for use with dilute cleaning solutions having a water:ammoniumhydroxide:peroxide volume ratio of greater than about 5:1:1, preferablyabout 40:1:1.

In a further aspect of the invention, a blend of two or more complexantsof the invention is provided to provide enhanced SC-1 cleaning.Exemplary is the use of CDTA in combination with other aminoacetic acidssuch as DTPA (diethylenetriaminepentaacetic acid). It has been foundthat the CDTA has higher complex stability constants for calcium, iron,copper, manganese, and most other light metals. DTPA has higherstability constants for lanthanides, actinides, and several other heavymetals (Hg, Tl, Bi). When the latter group is of concern, the use ofCDTA and DTPA in combination provides an enhanced cleaning solution.Since DTPA is not as stable as CDTA against oxidative decomposition,continuous or periodic replenishment with a DTPA solution is necessaryfor the extended use of this modified SC-1 solution.

In an additional aspect of the invention, aminoacetic-based complexantsare provided for use in the solutions and method of the invention whichhave improved resistance to oxidative degradation and enhanced bathstability for cleaning a plurality of semiconductor substrates. Broadlystated, the compounds comprise aminoacetic compounds in which anynitrogen atom is separated from its nearest nitrogen neighbor by 1-4carbon atoms, and in which one or both of the following conditionsexist: (a) carbons in the beta position relative to nitrogen carry nohydrogen atoms (e.g., 2,2-dimethyl-1,3-diaminopropane-N,N,N',N'-tetraacetic acid (DMDPTA) andmethylenediamine-N,N,N',N'-tetraacetic acid (MDTA)) or carry onlyhydrogens which are anticlinal or antiperiplanar to said nitrogen (e.g.,cis, cis, cis-3,5-dimethyl-1,2-diaminocyclopentane-N,N,N',N'-tetraaceticacid), and (b) the compound is polycyclic and any hydrogens in asynclinal or synperiplanar conformational relationship with an aminonitrogen are in bridgehead positions (e.g., cis-bicyclo (2.2.2)octane-2,3-diamine-N,N,N',N'-tetraacetic acid (CBODTA)).

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The semiconductor surface treating solution of the present inventioncomprises an aqueous solution which is alkaline and which containshydrogen peroxide. The treating solution contains an additive whichremoves and/or inhibits contamination of a plurality of semiconductorsurfaces being treated during the fabrication process by metallic ionspresent in the solution or on the semiconductor surface. Typically, thesolution is used until it is no longer useful to clean the surface and asolution is generally used for about 4-8 hours of actual cleaning.Make-up NH₄ OH or other base and/or additive may be added during theprocess to form a replenished solution. H₂ O₂ may also be replenished ifnecessary.

In general, the basic aqueous solution for treating the semiconductorsurface is termed SC-1 and contains an organic or inorganic base,hydrogen peroxide and water. In conventional treating solutions, thevolume ratio of water:base:peroxide is about 5:1:1 by volume.

With regard to the base material of the composition, a variety oforganic or inorganic bases may be used. Exemplary are ammoniumhydroxide, tetramethylammonium hydroxide or choline(2-hyroxyethyltrimethylammonium hydroxide) and the like. Hydrogenperoxide is the preferred oxidant and, as noted above, is preferablyused in the form of a 28-32%, by weight, aqueous solution.

With regard to the treating solution, the volume ratio of water:ammoniumhydroxide:peroxide is conventionally about 5:1:1. It is an importantaspect of the invention that dilute solutions of the aqueous cleaningsolution can be used with ratios up to 200:x:1 and above wherein x isabout 0.05 to 2. These solutions have been found to provide enhancedoperating benefits when used with the complexants of the invention toremove and/or inhibit metallic contamination of the semiconductorsubstrates being treated.

With regard to the conventional treating solutions having a H₂ O:NH₄OH:H₂ O₂ volume ratio of about 5:1:1, it has been found that the use ofCDTA as an additive in an amount greater than 1 ppm inhibitscontamination of the semiconductor surface by metallic ions present inthe solution or on the substrate surface. Previous disclosures regardingthe use of CDTA, for example, in U.S. Pat. No. 5,290,361, show the useof CDTA in an amount of 1 ppm and discloses that this amount wasrelatively ineffective when compared to the phosphonic acid chelaters ofthe patent. The CDTA was also compared with EDTA, TTHA and NTA and allwere considered to be ineffective. It has been found that amounts ofCDTA greater than 1 ppm, preferably 10-100 ppm are effective in such5:1:1 solutions and that the solutions have enhanced stability whentreating a plurality of semiconductor substrates in a typicalfabrication process.

It is a further important aspect of the invention that the cleaningeffectiveness and stability of dilute SC-1 cleaning solutions containinggreater than 5:x:1 volume ratio of components to about 200:x:1 and abovemay be enhanced by addition of about 1 ppm to 1,000 ppm of the CDTAadditive preferably 10 ppm to 100 ppm. Likewise, these dilute solutionshave enhanced stability and provide effective chelating results forremoving and/or inhibiting contamination of the semiconductor surface bymetallic ions present in the cleaning solution or on the substratesurface.

The CDTA used in the invention is preferably the trans isomer. The cisisomer may also be used, but is less preferred as is a mixture of thetrans and cis isomers. The trans isomer is commercially available and itis contemplated herein that a cis/trans isomer mixture may be preparedusing the same or similar synthesis techniques as used to make a puretrans isomer or a pure cis isomer. It will be apparent to those skilledin the art that a variety of substituents can be on the cyclohexane ringof CDTA without impairment or even with some improvement of theeffectiveness of the complexant. In particular, alkyl substituents(e.g., C1-C4) in the 1, 2, 5 and/or 6 ring position(s) further stabilizethe complexant against oxidative decomposition and enhance the bathstability and effectiveness.

It is also contemplated in the invention that the CTDA can be used incombination with other chelating agents such as EDTMP to provideenhanced synergistic treatment results. Other exemplary complexantswhich may be used with CTDA include nitrilotris (methylene)triphosphonic acid (NTMP).

It is another important feature of the invention that oxidationresistant aminoacetic complexants may be used either alone or incombination with the CTDA to provide an enhanced stable SC-1 cleaningsolution. These aminoacetic based complexants are generally defined ashaving carbons at beta positions relative to nitrogen atoms which carbonatoms carry no hydrogen atoms or carry only hydrogens which areanticlinal or antiperiplanar to said nitrogens. One class of complexantshas improved resistance to oxidative degradation by the cleaningsolution and provides enhanced stability when treating a plurality ofsemiconductor substrates and other operating effects. Three examples ofsuch aminoacetic compounds are (DMDPTA)2,2-dimethyl-1,3-diaminopropane-N,N,N',N'-tetraacetic acid and (MDTA)methylenediamine-N,N,N',N'- tetraacetic acid and cis, cis,cis-3,5-dimethyl-1,2-diaminocyclopentane-N,N,N',N'-tetraacetic acid. Anadditional class of such oxidation-resistant aminoacetic compoundsincludes polycyclic compounds in which any hydrogens in a synclinal orsynperiplanar conformational relationship with an amino nitrogen are inbridgehead positions. A typical example of this type of aminoaceticcompound is cis-bicyclo (2.2.2) octane-2,3-diamine-N,N,N',N'-tetraaceticacid (CBODTA).

The SC-1 cleaning solutions are generally used at elevated temperaturesof about 65-75° C. for about 5-20 minutes, typically 10 minutes for eachbatch of wafer cleaned. Usually 25-50 wafers are treated per batch in abath and mixing of the batch is generally by recirculation of the bathsolution.

The present invention is illustrated by the following examples. In theexamples, the composition of the treating solution will be referred toas a volume ratio of water:base:peroxide which individual components aremade using commercial solutions such as a 25-30% ammonium hydroxide and28-32% hydrogen peroxide. Unless otherwise specified, all otherpercents, ppm and ppb are by weight.

EXAMPLE 1

The stability of complexants in an SC-1 treatment solution was tested bytheir effectiveness at suppressing H₂ O₂ decomposition by measuring theconcentration of hydrogen peroxide remaining in a heated (65° C.)20:0.5:1 SC-1 solution after 10 hours. The SC-1 solutions werepre-contaminated with Al, Ca, Co, Cu, Fe, Mn, Ni, Ti and Zn all withconcentrations about 1 ppb. The initial concentration of peroxide was0.44 M.

                  TABLE 1    ______________________________________                                     % H.sub.2 O.sub.2    RUN   COMPLEXANT (PPM) H.sub.2 O.sub.2 (M)                                     REMAINING    ______________________________________    1     CDTA (20)        0.48      109    2     CDTA (10)        0.47      107    A     NONE             0.09      20    B     EDTMP (20)       0.39      89    C     EDTMP (25)       0.40      91    D     EDTA (20)        0.10      23    E     EDTA (17)        0.10      23    ______________________________________

The reason for a higher hydrogen peroxide concentration in Runs 1 and 2compared to the initial concentration, is due to evaporation of theother solution components such as water and ammonia. All the other runswould otherwise be comparative. EDTMP is ethylenediamine tetra(methylenephosphonic) acid and EDTA is ethylenediaminetetraacetic acid.

EXAMPLE 2

The stability of the CDTA complexant in an 20:0.5:1 solution containing9.3 ppm CDTA in which a silicon wafer was immersed was determined bymaintaining the solution at a temperature of about 63° C. and measuringthe CDTA concentration by ion chromatography. The results are shownbelow:

                  TABLE 2    ______________________________________    TIME (HOURS)  CDTA (PPM)  % CDTA LOSS    ______________________________________    0             9.2 ± .3 --    1             8.7 ± .2 5.4    2             8.4 ± .3 8.7    3             7.9 ± 0.05                              14.1    4             7.6 ± 0.02                              17.4    ______________________________________

From the above results, it can be seen that CDTA is substantially stablein the SC-1 solution over a period of 4 hours during treatment of asilicon wafer. This is important for process control where time can beused to determine when the SC-1 solutions should be replaced orreplenished.

EXAMPLE 3

Example 2 was repeated except that the solution temperature was 65° C.,and Cu⁺² and Fe⁺³ ions were added as indicated to determine their effecton the stability of CDTA. The results are as follows:

    ______________________________________    COMPOSITION    % CDTA LOSS/HOUR    ______________________________________    No added metal ions                   4.55    10 ppb Cu.sup.+2                   5.35    10 ppb Fe.sup.+3                   6.04    ______________________________________

The results indicate only a minor effect of the metal ions on thestability of the CDTA. This effect is normally insignificant underregular process conditions, where Cu⁺² and Fe⁺³ concentrations are muchlower than 10 ppb.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

Thus, having described the invention, what is claimed is:
 1. A methodfor treating a plurality of batches of semiconductor substrate surfaceswith the same cleaning solution to clean the surfaces while removingand/or inhibiting metallic contamination of the treated surface with themethod comprising contacting a plurality of batches of the semiconductorsubstrate surfaces with an aqueous solution comprising an organic orinorganic base, hydrogen peroxide and 1,2-cyclohexane diaminetetraaceticacid in an amount greater than about 1 ppb to about 1,000 ppm.
 2. Themethod of claim 1 wherein the 1,2-cyclohexane diaminetetraacetic acid isin an amount of about 10 ppm to 100 ppm.
 3. The method of claim 1wherein the 1,2-cyclohexane diaminetetraacetic acid is in the transisomer form.
 4. The method of claim 1 wherein the solution has awater:base:peroxide volume ratio of about 5:x:1 to 200:x:1 wherein x isabout 0.025 to
 2. 5. The method of claim 1 wherein the solution isreplenished during use.
 6. The method of claim 1 wherein 1,2-cyclohexanediamindetraacetic acid is added in the form of a solution in ammoniumhydroxide.
 7. An aqueous solution for treating a plurality of batches ofsemiconductor substrate surfaces comprising an organic or inorganicbase, hydrogen peroxide and 1,2-cyclohexanediaminetetraacetic acid as anadditive in an amount greater than 1 ppm to 1,000 ppm to remove and/orinhibit contamination of the semiconductor surface by metallic ionspresent in the treating solution or on the semiconductor substratesurface.
 8. The aqueous solution of claim 7 wherein the CDTA is in anamount of about 10 ppm to 100 ppm.
 9. The aqueous solution of claim 7wherein the solution has a water:ammonium hydroxide:peroxide volumeratio of about 5:x:1 to 200:x:1 wherein x is 0.025 to 2.